I. Field of the Invention
The present invention relates generally to devices and methods used to monitor neurophysiological conditions in humans and animals, and more particularly to such devices and methods as applied to electroencepholographic (EEG), electromyographic (EMG), electrocardiographic (ECG), evoked potential (EP), and nerve conduction velocity (NCV) studies.
II. Description of Prior Art
Electromyography (EMG), electroencephalography (EEG), electrocardiography (ECG) and evoked potential (EP) measurement (collectively known as neurophysiology) have undergone rapid development in the last ten to fifteen years, requiring increasingly sophisticated machines with significant computing power and specialized controls and input devices. This has led to large devices which have become very expensive and generally immobile. Those devices which are labelled as portable are still relatively heavy, very expensive, and do not offer the computing power of their larger cousins. Fortunately, the development of personal computers, particularly laptop computers, has outpaced EMG and EEG machines, making the below-described invention possible. The latest generation of portable laptop computers has computing capability equal to or greater than most EMG and EEG machines available today. In addition, with the advent of digital signal processing, many of the specialized controls needed for running the latest machines can now be incorporated into software, so that a keyboard and mouse (or trackball) are often all that is needed to control the processes of the testing equipment. Therefore, this invention provides a solution to the problems with most of the equipment available today for such testing, namely that such equipment is bulky, expensive, complicated and sometimes difficult to obtain.
Prior to describing the preferred embodiments of the present invention, a short explanation of common neurophysiological techniques and testing is provided. Neurophysiological signals are generated by the electrical discharge of neurons in the central or peripheral nervous system or muscle fibers. The signal may occur due to voluntary or involuntary activity, or may be induced by direct stimulation from an external source. To record a neurophysiological signal, an electrode is placed at or near the nerve or muscle generating the signal (the active site), and another electrode is placed distant from the site (the inactive site). A ground electrode is also placed somewhere on the patient's body, and all three electrodes are connected to the detection instruments. The electrical discharges of nerves and muscles can be observed by changes in the voltage of the active electrode relative to the inactive electrode. To measure this effect, the signals from both the active and inactive electrodes are typically amplified and passed to a differential amplifier and/or an analog-to-digital converter, which will produce the signal corresponding to the voltage difference between the two electrodes. This signal is commonly displayed on an oscilloscope and sometimes "played" over a speaker device.
Electromyography (EMG) requires the least amount of hardware to obtain a measurable signal, because only the three electrodes described earlier are used, and no stimulating device are required. A ground electrode (G0) is placed on the skin, while the active electrode (G1) is inserted into the muscle being tested, and the inactive electrode (G2) is placed on the skin near that muscle in a monopolar array. With bipolar needles, G1 is an insulated wire travelling down a barrel which serves as G2, as is known to those of ordinary skill. The leads which connect the electrodes to the EMG machine are standardized, being either 2 mm plugs or a 6-pin DIN plug. More sophisticated EMG techniques, such as single fiber EMG or macro EMG differ in the type of needle used and the software needed to drive them. However, the inputs, preamplifiers, and analog-to-digital (A/D) converters required to process the response signals are the same.
Electroencephalography (EEG) is similar to EMG in that it records a voluntary or involuntary signal without stimulation. An array of electrodes, typically 21 in number, is placed at standardized positions on the scalp. EEG tracings are obtained by comparing the signals from one electrode with another. Most commonly, a tracing is obtained by comparing the signal from one electrode to the signal from the electrode next to it (called a "bipolar montage"). Traces may also be obtained by comparing the signals from several electrodes to a common reference electrode (called a "referential montage"). Generally, 16 traces are recorded at once, but this is limited only by the number of possible comparisons between electrodes.
For nerve conduction velocity (NCV) studies, stimulating electrodes (S1 and S2) are also required in addition to the ground (G0) and the pickup electrodes (G1 and G2). The stimulating electrodes are most commonly present on a hand-held stimulator device. An electric shock is applied over the nerve to be tested, and the signal is picked up along another segment of the nerve or is picked up at a muscle supplied by that nerve. The latency and amplitude of the signal is measured, and the process is repeated using another point of stimulation. With these multiple points of stimulation, conduction velocities can be obtained in each segment of the nerve. Some special nerve conduction tests require a specially modified reflex hammer which delivers a timing (or "triggering") signal to the receiving electronics when the operator applies the stimulus.
Evoked potential (EP) methods also require both stimulation and pickup. The arrangement of pickup electrodes varies depending on the type of EP method employed by the practitioner. However, scalp electrodes are typically placed as in an EEG test procedure. In addition, electrodes can be placed on the neck or at Erb's point (over the brachial plexus in the shoulder). The stimulus required for EP depends on the system to be tested, but can be visual (generally requiring a video display or goggles), auditory (requiring earphones or a similar auditory headset), or electrical (to stimulate sensory nerves as in nerve conduction studies). The stimulus is given repeatedly, and the signals recorded with each stimulus are averaged to reduce random background nerve activity.
The present invention is a device which may be connected to one of the input ports of a standard desktop or laptop computer, such as a serial, parallel or SCSI port. The invention contains the specialized inputs needed to record EMG and other analog neurophysiological signals. These analog signals can be digitized and processed by the laptop computer using preloaded software. The many advantages over currently available systems are readily apparent. First, such a device would dramatically lower the price of neurophysiology testing machines, because standard computer equipment is employed to control and process much of the data. Second, it will also enable manufacturers of current EMG and EEG equipment to devote greater resources to the improvement of controlling software as a complement to ongoing efforts in hardware design. Third, upgrades in hardware used for such testing will be much cheaper and easier for end users, because it would merely require the purchase of commercially available computer equipment. Fourth, the invention is highly portable because of its modular design, which is advantageous for those who wish to perform EMG and other neurophysiological tests at more than one place, or for those who perform many studies in intensive care units.